An Taisce says it may consider legal action to prevent the Seanad moving in to the National Museum while repair works gets under way at Leinster House.

The trust has submitted an objection to the plans to the planning department of Dublin City Council. It has called for sittings of Dáil Eireann to be rescheduled to allow senators to use that chamber instead.

The National Museum building is a protected structure.

The proposed works include the occupation of at least one sizable gallery which constitutes a change of use. An Taisce is also upset at other changes, including the breaching of the external wall of the museum to facilitate entrances for senators and the public. They also object to a lift being added to the building.

All of the changes have been proposed without planning permission being sought because the relevant authorities are seeking exemptions. An Taisce says the move is 'disgraceful'.

"'Exempted Development' is only permissible if 'the works would not affect the character of the structure'," said Dr. Mark Clinton of An Taisce's Monuments and Antiquities Committee. "Clearly the works envisaged will alter the character of the protected structure and permission for the works should not be given," he added.

Dr Clinton also questioned an estimated €1.7m spend on the work. An Taisce works to preserve some of the nation's most historic and culturally important buildings. Its communications director Charles Stanley-Smith said the State had a long history of changing buildings, including Leinster House, on a site which was originally designated as a cultural campus 200 years ago.

"And now, in 2016, they have set their sights on part of The National Museum," he added.

"Another 'temporary' acquisition, this time for the Seanad. We will consider a legal move if this goes ahead and go to court if necessary."